1. Field of the Invention
The present invention relates in general to a charge coupled device (hereinafter referred to as a CCD) image sensor, and more particularly to a CCD image sensor having an improved structure of a vertical charge coupled device (hereinafter referred to as a VCCD) region thereof.
2. Description of the Prior Art
A solid state image scanner is generally formed by disposing a plurality of photodetectors and a plurality of photoscanners on a substrate of a semi-conductor material, such as silicon oxide. Elements may be selected for the photodetectors in order to establish an image pickup from the visible range to the infrared range.
For example, the solid state image scanner may include an MOS switch, a CCD, etc. The CCD is comprised of a vertical charge coupled device (VCCD) region and a horizontal charge coupled device (hereinafter referred to as an HCCD) region.
A CCD image sensor utilizing CCDs typically converts light energy through the photodetector into an electric signal, with the photoelectric efficiency being very low in a wavelength of a red color type, wherein the amount of light energy is small. That is, image signal charges of the red color type are not transferred fully to the VCCD region, resulting in difficulty in providing a perfect picture.
With reference to FIG. 1 there is shown a schematic diagram of a construction of a conventional CCD image sensor of an interline transfer type. As shown in this figure, a conventional CCD image sensor comprises an N type horizontal charge coupled device (HCCD) region 3 and a plurality of N type vertical charge coupled device (VCCD) regions 2, to each of which a series of N type photodiodes 1 are connected in a one to one relationship. Each of the N type photodiodes 1 is connected to the N type VCCD region 2, such that an image signal charge output from the N type photodiodes 1 is transferred to the N type VCCD region 2 in a single direction. Also, the N type VCCD regions 2 are connected to the N type HCCD region 3, such that the signal charges transferred from the photodiodes 1 are transferred to the N type HCCD region 3 simultaneously in response to predetermined clock signals. To the output of the N type HCCD region 3 is connected a sensing amplifier 4 for sensing states of the signal charges and amplifying the sensed states of the signal charges by a predetermined amplification degree.
With reference to FIG. 2, there is shown a sectional view, taken on the line a--a' of FIG. 1. As shown in this figure, on an N type substrate 5 is formed a P type well 6, on which is formed the N type photodiode 1 being spaced apart at a desired interval from the N type VCCD region 2. A transfer gate electrode 7 is formed on the upper side of and between the N type photodiode 1 and the N type VCCD region 2, and a gate electrode 8 is formed on the N type VCCD region 2. As illustrated in FIG. 2, a P.sup.+ type ion layer 9 is formed underneath and surrounding the N type VCCD region 2.
The P.sup.+ type ion layer 9 is adapted to prevent noises, such as a blooming, occurring when excess electronic signal charges resulting from a smear are transferred to the N type VCCD region 2.
Also, on the surface of the N type photodiode 1 is formed a thin P.sup.+ type ion layer 10 for applying an initial bias to the surface.
Preferably, materials of the transfer gate electrode 7 and the gate electrode 8 may be polysilicons. Although not shown, insulating films are formed between the transfer gate electrode 7, the gate electrode 8 and the surface of the P type well 6. This technique is shown in pages 1448, 1451, 1458 and 1498 of "Transactions on Electron Devices" published by IEEE, August, 1985.
With reference to FIG. 3, there is shown a potential profile diagram, taken on the line b--b' of FIG. 2, illustrating a vertical potential distribution under the condition that the charges e are contained in the N type VCCD region 2.
The image signal charges e are moved from the N type photodiodes 1 to the N type VCCD region 2, upon application of a high voltage to the transfer gate electrode 7. At this time, the P.sup.30 type ion layer 9 formed between the N type VCCD region 2 and the P type well 6 prevents the P type well from being fully depleted, resulting in a higher potential barrier. As a result, the excess charges resulting from a smear are not moved to the N type VCCD region 2.
With reference to FIG. 4, there is shown a potential profile diagram, taken on the line c--c' of FIG. 2, illustrating a state in which the P type well 6 is fully depleted due to physical properties of the PN junction upon the application of a high voltage to the N type substrate 5, resulting in a lower potential barrier.
If strong light energy is transmitted to the N type photodiode 1, such that the signal charges produced from the N type photodiode 1 are too great, excess charges e' are transferred to the N type substrate 5 over the P type well 6. Here, the depth of the P type well 6 between the N type photodiode 1 and the N type substrate 5 is shallow, so that the excess charges e' are not moved to the N type VCCD region 2, but are readily absorbed into the P type well 6. Thus, the P type well is adapted to prevent an overflow drain (OFD). To achieve this effect, the portion of the N type substrate 5 under the N type photodiode 1 may be formed higher than others and the portion of the N type photodiode 1 may be formed deeper. At this time, since a desired potential barrier resulting from the P.sup.+ type ion layer 9 is formed in the N type VCCD region 2, as shown in FIG. 3, the excess charges e' from the N type photodiode 1 are not transferred to the N type VCCD region 2.
However, this conventional CCD image sensor has disadvantages as follow:
First, although the P.sup.+ type ion layer 9 formed underneath and surrounding the N type VCCD region 2, as shown in FIG. 2, has an effect of preventing a blooming resulting from a smear to a degree, the excess charges cannot be moved fully to the N type substrate 5, thereby causing the excess charges to be moved to the N type VCCD region of another cell, resulting in the blooming.
Second, because the P type well 6 is formed shallow in the portion between the N type photodiode 1 and the N type substrate 5 for ease of absorption of the excess charges e', the image signal charges of the red color type of long wavelength are not transferred fully to the N type VCCD region 2, but are partially absorbed into the N type substrate 5.
Third, it is difficult to provide a substrate which is etched in part in order to meet the above conditions.